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Lot2Learn (Structural)
17 May 11 23:16
Do grade beams have to be deep enough to have bottom/beam below frost depth? Since they are not actually supported by the soil but rather by pile caps, so I was just wondering.
JAE (Structural)
18 May 11 9:02
Yes - if soil can freeze and heave, it will lift the grade beam upward and damage it.  Frost heave can be very powerful.

You possibly could design for this upward force, but there would be very high uncertainty as to its magnitude.

 
Lot2Learn (Structural)
18 May 11 11:31
Thanks JAE, that makes sense.
dik (Structural)
18 May 11 13:51
You can put a void material beneath them... I typically spec, 'Frost Cushion'... it's a plastic material void form and I've had good results with it for years.  We typically have 6' of frost penetration... and often have 24" or 30" deep grade beams on piles... Put the void under the pile cap if you have these... we often extend the pile rfg into the grade beam (or dwls) and don't have a pile cap...

Dik
msquared48 (Structural)
18 May 11 15:02
If the grade beams are baelow a heated space though, I would not expect any frost.  Only at the peripheral grade beams.  

After a few years, the soil may settle away from the underswide of the grade beams, making them less susceptible to frost heave damage.  But you have no direct control over that and cannot rely on it.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

kieran1 (Structural)
18 May 11 16:08
As Dik says you can install materials to act as a frost cushion. I try to postion beams at a depth which allows building services to get out without passing through the beam. Unless your frost depth is very deep, this usually decides the level of the beam

Kieran
 

csd72 (Structural)
19 May 11 9:00
JAE has covered it well.

I would be hesitant to use a void underneath as this could fill up with water and still create uplift forces when this freezes.

The additional depth can pay dividends in other ways such as removing the need for shear reinforcement in the grade beam and thus vastly reducing labour.
dik (Structural)
19 May 11 11:56
csd72... that's the reason for using Frost Cushion... it doesn't have voids to fill with water and has a low compressive strength.  I've done reports of failures caused by the cardboard void filling with water and heaving... pressures from ice can be 1000 psi... and greater.

Dik
BAretired (Structural)
23 May 11 23:15
I have used frost cushion without problems, but a high water table  could fill the voids with water and freeze resulting in frost heave of the grade beam.  

http://www.northlandconstruction.com/product/frost-cushion/

I prefer a cushion which occupies the entire void space.  It compresses when necessary, then bounces back to its original volume.  There may be other products which satisfy this requirement, but GeoSpan is the one I have specified recently.  

http://www.plastifab.com/applications/geotechnical/compressible_fill/geospan.html

BA

JAE (Structural)
23 May 11 23:22
BAretired - have you ever used these cushions for expansive clay conditions?  Used to go to a lot of trouble to create voids with concrete fill retainers under grade beams.  Just wondering if this would work in that sort of condition.

 
BAretired (Structural)
23 May 11 23:59
JAE.

We have expansive clays in our area, though not nearly as expansive as the soils found in the neighboring provinces of Saskatchewan and Manitoba where heaving of six or seven inches has been reported.

A cushion will not function adequately unless its thickness is at least twice the expected heave due to either frost or swelling soil or, alternatively, unless the grade beam is reinforced for the uplift pressure and is tied down at supports to resist the negative reactions.

I typically use 4" GeoSpan in the Edmonton area with no problems so far.   For a project in Winnipeg, Manitoba, I used 6" GeoSpan and have heard of no problems to date.  Grade beams were designed for a substantial uplift pressure in the event that the soil swelling exceeded the estimates.  This could be considered double kill but it didn't really matter because the cost of the reinforcement was not significant.

In the 1950's, it was common to form grade beams with a 'V' shape on the bottom with the idea that the clay soil would cleave to each side when it heaved, hence prevent uplift.  I never could understand that logic, but I cannot criticize it too much because some of those old buildings are still around today and are behaving reasonably well.

BA

dik (Structural)
24 May 11 7:28
I've used both the geospan and their equivalent for grade beams (geogrid, I think). I've not had problems with frost cushion, either. 30 years ago, I used to use aerofoam 'skins', and although it has a higher compressive capacity, I've not had a problem with it.

I've done a lot of warehouses and pre-eng buildings using 24" or 30" grade beams using that approach and not had a problem.

JAE:
The Winnipeg area has hightly plastic clays for a depth of 30-40' typically. Fortunately, the soil is generally saturated and generally desicates shrinks a bit after construction.

Dik

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